Where Sea Turtles Spend Their 'Lost Years'

It must be hard for mama loggerhead turtles. After their eggs hatch, their little offspring scurry down the beach, where many of them are snapped up by hungry birds. Even more get eaten by fish. To top it off, the survivors promptly head out to sea and vanish. They never write; they never call.

Now, researchers have published the first satellite tracking data from young sea turtles, charting a leisurely voyage under the sun. The youngsters idly float north along the Gulf Stream, then head toward the Azores. Along the way, they spend most of their time riding the waves, and the sun helps raise their body temperature. “The data from this handful of turtles could lead to some really important developments” in understanding turtle populations, says Nathan Putman, a turtle biologist at Oregon State University, Corvallis.

The early life of loggerheads and other sea turtles used to be called “the lost years,” because no one knew exactly where they went. But over decades, researchers pieced together the animals’ natural history from ship observations, the pattern of ocean currents, and other data. More recently, other scientists have studied isotopes from turtle tissue to analyze their diet and past locations and created computer simulations of their journey. The basic picture is this: Hatchlings head out to sea to avoid fish, sharks, and other predators. Once off the continental shelf, they eventually end up in a current, called the North Atlantic subtropical g. A few years later, they come back to their birthplaces on the East Coast of the United States.

Satellite tags have helped researchers track adult turtles. But the brick-sized transmitters were too big for young turtles. As the technology became smaller, marine biologist Katherine Mansfield of the University of Central Florida, Orlando, was able to use transmitters the size of smartphones. Solar cells helped reduce the size and weight by requiring fewer batteries. Mansfield and her colleagues collected 17 loggerhead hatchlings, each about 3 to 4 centimeters in diameter, from beaches in southeast Florida. They raised them in the lab until they were 3 to 9 months old—big enough to glue on the transmitters—and released them from a boat about 18 kilometers offshore.

The transmitters survived between 27 and 220 days, during which some turtles roamed as far as 4300 kilometers, the team reports today in the Proceedings of the Royal Society B. As expected, the turtles all headed north with the Gulf Stream, then most turned eastward around Cape Hatteras in North Carolina. The paths were consistent with what Putman’s computer models had predicted. The turtles weren’t single-mindedly heading toward the Azores, but looping around in small eddies and trying to avoid the coldest water. They also spent almost all of their time on the surface, and Mansfield suspects that this is to help stay warm. “It’s kind of a common sense thing,” Mansfield says. (They may also be waiting to develop the lung capacity for diving.)

Seven turtles ventured into the Sargasso Sea, inside the gyre. In addition to hosting crustaceans and other food, the dark plant matter there absorbs warmth from the sun. The transmitters showed that the temperature around the turtle shell was 4°C to 6°C warmer than otherwise expected. “They’re absorbing a lot of sunlight,” Mansfield says. That’s exciting other turtle researchers. “This is the first data to document the thermal benefits of surface living for oceanic juvenile sea turtles,” says Alan Bolten of the Archie Carr Center for Sea Turtle Research at the University of Florida in Gainesville.

The results are a “fairly clean and clear demonstration” of where loggerheads go while young, Putman says. And by showing how temperature can vary, they raise the question of how various migration paths might affect turtle growth and even population dynamics, he adds. For example, turtles that get to the warmth of the Azores might mature more quickly than those that hang out in cooler waters elsewhere. Bolten cautions that the turtles in the study are bigger than hatchlings, but Mansfield says she expects their movements would be similar.